The field of the present invention is generally related to computerized tomography (CT) and, particularly, to apparatus for high data rate communication in a CT system.
CT systems typically employ a rotating frame or gantry to obtain multiple x-ray images, or views, at different rotational angles. Each set of images is referred to in the art as a "slice". A patient or inanimate object is generally positioned in a central opening of the rotating frame on a table which is movable axially, thus enabling respective slices to be obtained at multiple axial positions as well. Each of the slices obtained is then processed in a computer according to predetermined algorithms to produce enhanced images for diagnostic or inspection purposes.
The rotating frame includes an x-ray source, a detector array and electronics necessary to generate image data for each view. A set of stationary electronics is employed for processing raw image data into the enhanced form. Thus, it is necessary to provide for communication of the image data between the rotating frame and a stationary frame of the CT system.
The data rate for communication between the stationary and rotating frames is an important factor because it is desirable to obtain the desired views as fast as possible to reduce patient discomfort and/or to maximize equipment utilization. In current CT systems, a single view typically comprises about 800 detector channels with a 16 bit representation for each individual detector channel output (i.e., 12.8 Kbits per view) and is typically repeated 1,000 times per second, yielding a net data rate requirement of approximately 13 Megabits per second (Mbit/sec) for image data alone. Future CT systems capable of simultaneously constructing multiple image slices by employing four, eight, or sixteen times as many detector channels will increase the data rate requirement to beyond 150 Mbit/sec for image data alone.
In order to provide a communications link with the requisite data rate, some prior CT systems have employed an umbilical cable connected to the rotating gantry. One or more flexible, shielded coaxial cables are used in the umbilical cable for high speed communications, and other conductors are used for power and discrete control signals. The umbilical cable is typically capable of plus or minus 360.degree. of rotation, so that the gantry is limited, for example, to a total of 720.degree. of rotation. In operation, the gantry is accelerated to a desired rotational speed and the desired views are taken before the 720.degree. limit is reached. Near the 720.degree. limit, the gantry is decelerated to a stop, and then accelerated in the reverse direction to acquire more views. The gantry thus cycles back and forth within the 720.degree. limit.
Such "cycling" type CT systems have two main disadvantages. One disadvantage is that the decelerating and re-accelerating of the gantry is fairly time consuming. The gantry, with all equipment in place, is both large and massive, so that even with large motors, the time consumed in accelerating the gantry can be substantial. The second disadvantage is somewhat of a corollary to the first, in that the need to repeatedly accelerate such a large mass produces a large amount of mechanical stress and wear.
Another type of CT system is known in the art in which brushes and slip rings are utilized for electrically linking the rotating frame to the stationary frame. In these systems, the rotating frame or gantry is free to rotate continuously, eliminating the need for the above described back-and-forth movement of the gantry and thereby providing greater flexibility for the acquisition of the desired views. However, prior CT systems utilizing brushes and slip rings for communications have generally suffered from significant limitations in the data rates which can be achieved. This is due to the substantial time required to propagate the signals around the circular slip rings. At the desired data rates, the electrical path length around the rings is an appreciable fraction of a bit period, so that electromagnetic waves propagating around the rings in opposite directions may arrive at a reception point at substantially different times in a bit period, causing garbled reception.
U.S. Pat. No. 5,208,581 issued to A. K. Collins, assigned to the assignee of the present invention and herein incorporated by reference, is another type of gantry in which brushes and slip rings are employed for communication. Although the design of Collins provides relatively high speed communication between the stationary and rotating frames, the fact remains that the use of contacting brushes and rings inherently carries certain disadvantages. For example, the mechanical contact between the brushes and rings causes wear which requires such brushes and rings to be periodically replaced in order to maintain reliable communication. Furthermore, the design of Collins does not support the higher data rates needed for multiple-slice CT systems.
Other CT systems have employed an optical data link for communication between the stationary and rotating frames. Although an optical data link design avoids typical drawbacks of slip rings and brushes, such optical design requires optics which must be fabricated under tight specifications and which in operation require substantial spatial alignment in order to achieve reliable optical coupling along the relatively long circumference of the rotating frame. This leads to high costs and, thus, it is desirable to provide in a CT system an improved communication link which at a low cost provides reliable high data rate communication between the stationary and rotating frames of the CT system.